Public anger in China at dangerous levels of air pollution, which blanketed Beijing in acrid smog, spread Monday as state media queried official transparency and the nation's breakneck development.

The media joined Internet users in calling for a re-evaluation of China's modernisation process, which has seen rapid urbanisation and dramatic economic development at the expense of the environment.

Dense smog shrouded large swathes of northern China at the weekend, cutting visibility to 100 metres (yards) in some areas and forcing flight cancellations. Reports said dozens of building sites and a car factory in the capital halted work as an anti-pollution measure.

Doctors at two of the city's major hospitals said the number of patients with respiratory problems had increased sharply in the past few days, state media reported.

"Now it has been dark with pollution for three days, at least people are starting to realise how important the environment is," said one posting on China's Twitter-like Sina Weibo.

At the height of the smog Beijing authorities said readings for PM2.5 -- particles small enough deeply to penetrate the lungs -- hit 993 micrograms per cubic metre, almost 40 times the World Health Organisation's safe limit.

Experts quoted by state media blamed low winds, saying fog had mixed with pollutants from vehicles and factories and had been trapped by mountains north and west of Beijing. Coal burning in winter was also a factor, they added.

In an editorial Monday the state-run Global Times called for more transparent figures on pollution and urged the government to change its "previous method of covering up the problems and instead publish the facts".

Officials in China have a long history of covering up environmental and other problems.

Earlier this month a chemical spill into a river was only publicly disclosed five days after it happened, and authorities were widely criticised for initially denying the Severe Acute Respiratory Syndrome (SARS) outbreak in 2003.

Official PM2.5 statistics have only been released for China's biggest conurbations since the beginning of last year, and expanded to cover 74 cities earlier this month.

The tightly-controlled media has previously raised concerns over health problems linked to industrialisation. Observers say the statistics' increasing availability has forced them to confront the issue more directly.

The Xinhua state news agency criticised the "pollutant belt" that had spread across the country and warned that the authorities' stated goal of building a "beautiful China" was in jeopardy.

"A country with a brown sky and hazardous air is obviously not beautiful," it said.

"The environmental situation facing the country will be increasingly challenging," it said. "There is no reason to be too optimistic."

On Monday the Ministry of Environmental Protection announced measures to tackle the problem.

It pledged to limit vehicle exhaust emissions and promote the use of clean energy as well as step up the development of public transport systems in urban areas, state news agency Xinhua said.

The environmental watchdog also asked local authorities to increase their analysis of air pollution and publicise the results quickly as part of an early warning system for air quality, Xinhua reported.

Smog levels eased in the capital Monday, with the national monitoring centre putting the PM2.5 AQI figure at 183, or "light pollution", in the evening -- although the US embassy gave it a "hazardous" 335.

Levels remained high in many parts of China, with PM 2.5 AQI standing at 405 in Zhengzhou south of Beijing and 342 in Xian to the southwest.

Share prices of environment-related companies surged, with face mask producer Shanghai Dragon soaring by its 10 percent daily limit.

The smog dominated discussion on Sina Weibo. "This pollution is making me so angry," said one user, posting a picture of herself wearing a face mask.